Managing Tablespace Alerts

Oracle Database provides proactive help in managing disk space for tablespaces by alerting you when available space is running low. Two alert thresholds are defined by default: warning and critical. The warning threshold is the limit at which space is beginning to run low. The critical threshold is a serious limit that warrants your immediate attention. The database issues alerts at both thresholds.

There are two ways to specify alert thresholds for both locally managed and dictionary managed tablespaces:

By percent full

For both warning and critical thresholds, when space used becomes greater than or equal to a percent of total space, an alert is issued.

By free space remaining (in kilobytes (KB))

For both warning and critical thresholds, when remaining space falls below an amount in KB, an alert is issued. Free-space-remaining thresholds are more useful for very large tablespaces.

Locally managed tablespace—When you create a new locally managed tablespace, it is assigned the default threshold values defined for the database. A newly created database has a default of 85% full for the warning threshold and 97% full for the critical threshold. Defaults for free space remaining thresholds for a new database are both zero (disabled). You can change these database defaults, as described later in this section.

Dictionary managed tablespace—When you create a new dictionary managed tablespace, it is assigned the threshold values that Enterprise Manager lists for "All others" in the metrics categories "Tablespace Free Space (MB) (dictionary managed)" and "Tablespace Space Used (%) (dictionary managed)." You change these values on the Metric and Policy Settings page.

Note:

In a database that is upgraded from version 9.x or earlier to 10.x, database defaults for all locally managed tablespace alert thresholds are set to zero. This setting effectively disables the alert mechanism to avoid excessive alerts in a newly migrated database.

Setting Alert Thresholds

For each tablespace, you can set just percent-full thresholds, just free-space-remaining thresholds, or both types of thresholds simultaneously. Setting either type of threshold to zero disables it.

The ideal setting for the warning threshold is one that issues an alert early enough for you to resolve the problem before it becomes critical. The critical threshold should be one that issues an alert still early enough so that you can take immediate action to avoid loss of service.

When setting nonzero values for percent-full thresholds, use the greater-than-or-equal-to operator, OPERATOR_GE.

Restoring a Tablespace to Database Default Thresholds

After explicitly setting values for locally managed tablespace alert thresholds, you can cause the values to revert to the database defaults by setting them to NULL with DBMS_SERVER_ALERT.SET_THRESHOLD.

Modifying Database Default Thresholds

To modify database default thresholds for locally managed tablespaces, invoke DBMS_SERVER_ALERT.SET_THRESHOLD as shown in the previous example, but set object_name to NULL. All tablespaces that use the database default are then switched to the new default.

Viewing Alerts

You view alerts by accessing the home page of Enterprise Manager Database Control.

Limitations

Threshold-based alerts have the following limitations:

Alerts are not issued for locally managed tablespaces that are offline or in read-only mode. However, the database reactivates the alert system for such tablespaces after they become read/write or available.

When you take a tablespace offline or put it in read-only mode, you should disable the alerts for the tablespace by setting the thresholds to zero. You can then reenable the alerts by resetting the thresholds when the tablespace is once again online and in read/write mode.

Managing Resumable Space Allocation

Oracle Database provides a means for suspending, and later resuming, the execution of large database operations in the event of space allocation failures. Therefore, you can take corrective action instead of the Oracle Database server returning an error to the user. After the error condition is corrected, the suspended operation automatically resumes. This feature is called resumable space allocation. The statements that are affected are called resumable statements.

How Resumable Space Allocation Works

The following is an overview of how resumable space allocation works. Details are contained in later sections.

A statement executes in resumable mode only if its session has been enabled for resumable space allocation by one of the following actions:

The ALTER SESSION ENABLE RESUMABLE statement is issued in the session before the statement executes when the RESUMABLE_TIMEOUT initialization parameter is set to a nonzero value.

The ALTER SESSION ENABLE RESUMABLE TIMEOUTtimeout_value statement is issued in the session before the statement executes, and the timeout_value is a nonzero value.

A resumable statement is suspended when one of the following conditions occur (these conditions result in corresponding errors being signalled for non-resumable statements):

Out of space condition

Maximum extents reached condition

Space quota exceeded condition.

When the execution of a resumable statement is suspended, there are mechanisms to perform user supplied operations, log errors, and query the status of the statement execution. When a resumable statement is suspended the following actions are taken:

The error is reported in the alert log.

The system issues the Resumable Session Suspended alert.

If the user registered a trigger on the AFTER SUSPEND system event, the user trigger is executed. A user supplied PL/SQL procedure can access the error message data using the DBMS_RESUMABLE package and the DBA_ or USER_RESUMABLE view.

Suspending a statement automatically results in suspending the transaction. Thus all transactional resources are held through a statement suspend and resume.

When the error condition is resolved (for example, as a result of user intervention or perhaps sort space released by other queries), the suspended statement automatically resumes execution and the Resumable Session Suspended alert is cleared.

A suspended statement can be forced to throw the exception using the DBMS_RESUMABLE.ABORT() procedure. This procedure can be called by a DBA, or by the user who issued the statement.

A suspension time out interval, specified by the RESUMABLE_TIMEOUT initialization parameter or by the timeout value in the ALTERSESSIONENABLERESUMABLETIMEOUT statement, is associated with resumable statements. A resumable statement that is suspended for the timeout interval wakes up and returns the exception to the user if the error condition is not resolved within the timeout interval.

A resumable statement can be suspended and resumed multiple times during execution.

What Operations are Resumable?

The following operations are resumable:

Queries

SELECT statements that run out of temporary space (for sort areas) are candidates for resumable execution. When using OCI, the calls OCIStmtExecute() and OCIStmtFetch() are candidates.

DML

INSERT, UPDATE, and DELETE statements are candidates. The interface used to execute them does not matter; it can be OCI, SQLJ, PL/SQL, or another interface. Also, INSERT INTO...SELECT from external tables can be resumable.

Import/Export

As for SQL*Loader, a command line parameter controls whether statements are resumable after recoverable errors.

DDL

The following statements are candidates for resumable execution:

CREATETABLE ... ASSELECT

CREATEINDEX

ALTERINDEX ... REBUILD

ALTERTABLE ... MOVEPARTITION

ALTERTABLE ... SPLITPARTITION

ALTERINDEX ... REBUILDPARTITION

ALTERINDEX ... SPLITPARTITION

CREATEMATERIALIZEDVIEW

CREATEMATERIALIZEDVIEWLOG

What Errors are Correctable?

There are three classes of correctable errors:

Out of space condition

The operation cannot acquire any more extents for a table/index/temporary segment/undo segment/cluster/LOB/table partition/index partition in a tablespace. For example, the following errors fall in this category:

The number of extents in a table/index/temporary segment/undo segment/cluster/LOB/table partition/index partition equals the maximum extents defined on the object. For example, the following errors fall in this category:

The user has exceeded his assigned space quota in the tablespace. Specifically, this is noted by the following error:

ORA-01536 space quote exceeded for tablespace string

Resumable Space Allocation and Distributed Operations

In a distributed environment, if a user enables or disables resumable space allocation, or a DBA alters the RESUMABLE_TIMEOUT initialization parameter, then the local instance is affected. RESUMABLE cannot be enabled remotely. In a distributed transaction, sessions on remote instances are suspended only if the remote instance has already enabled RESUMABLE on the instance or sessions at its site.

Parallel Execution and Resumable Space Allocation

In parallel execution, if one of the parallel execution server processes encounters a correctable error, that server process suspends its execution. Other parallel execution server processes will continue executing their respective tasks, until either they encounter an error or are blocked (directly or indirectly) by the suspended server process. When the correctable error is resolved, the suspended process resumes execution and the parallel operation continues execution. If the suspended operation is terminated, the parallel operation aborts, throwing the error to the user.

Different parallel execution server processes may encounter one or more correctable errors. This may result in firing an AFTER SUSPEND trigger multiple times, in parallel. Also, if a parallel execution server process encounters a non-correctable error while another parallel execution server process is suspended, the suspended statement is immediately aborted.

For parallel execution, every parallel execution coordinator and server process has its own entry in the DBA_ or USER_RESUMABLE view.

Enabling and Disabling Resumable Space Allocation

Resumable space allocation is only possible when statements are executed within a session that has resumable mode enabled.

Resumable space allocation is enabled for a session when the ALTER SESSION ENABLE RESUMABLE statement is executed, and the RESUMABLE_TIMEOUT initialization parameter is set to a non-zero value for the session. When the RESUMABLE_TIMEOUT initialization parameter is set at the system level, it is the default for an ALTER SESSION ENABLE RESUMABLE statement that does not specify a timeout value. When an ALTER SESSION ENABLE RESUMABLE statement specifies a timeout value, it overrides the system default.

Resumable space allocation is disabled for a session in all of the following cases when the ALTER SESSION ENABLE RESUMABLE statement is executed

The session does not execute an ALTER SESSION ENABLE RESUMABLE statement.

The session executes an ALTER SESSION DISABLE RESUMABLE statement.

The session executes an ALTER SESSION ENABLE RESUMABLE statement, and the timeout value is zero.

Note:

Because suspended statements can hold up some system resources, users must be granted the RESUMABLE system privilege before they are allowed to enable resumable space allocation and execute resumable statements.

Setting the RESUMABLE_TIMEOUT Initialization Parameter

You can specify a default system wide timeout interval by setting the RESUMABLE_TIMEOUT initialization parameter. For example, the following setting of the RESUMABLE_TIMEOUT parameter in the initialization parameter file sets the timeout period to 1 hour:

RESUMABLE_TIMEOUT = 3600

If this parameter is set to 0, then resumable space allocation is disabled even for sessions that run an ALTER SESSION ENABLE RESUMABLE statement without a timeout value.

You can also use the ALTER SYSTEM SET statement to change the value of this parameter at the system level. For example, the following statement disables resumable space allocation for all sessions that run an ALTER SESSION ENABLE RESUMABLE statement without a timeout value:

ALTER SYSTEM SET RESUMABLE_TIMEOUT=0;

Using ALTER SESSION to Enable and Disable Resumable Space Allocation

Within a session, a user can issue the ALTER SESSION SET statement to set the RESUMABLE_TIMEOUT initialization parameter and enable resumable space allocation, change a timeout value, or to disable resumable mode.

A user can enable resumable mode for a session with the default system RESUMABLE_TIMEOUT value using the following SQL statement:

ALTER SESSION ENABLE RESUMABLE;

To disable resumable mode, a user issues the following statement:

ALTER SESSION DISABLE RESUMABLE;

The default for a new session is resumable mode disabled.

The user can also specify a timeout interval, and can provide a name used to identify a resumable statement. These are discussed separately in following sections.

Specifying a Timeout Interval

A timeout period, after which a suspended statement will error if no intervention has taken place, can be specified when resumable mode is enabled. The following statement specifies that resumable transactions will time out and error after 3600 seconds:

ALTER SESSION ENABLE RESUMABLE TIMEOUT 3600;

The value of TIMEOUT remains in effect until it is changed by another ALTER SESSION ENABLE RESUMABLE statement, it is changed by another means, or the session ends. If the RESUMABLE_TIMEOUT initialization parameter is not set, then the default timeout interval when using the ENABLE RESUMABLE TIMEOUT clause to enable resumable mode is 7200 seconds.

Naming Resumable Statements

Resumable statements can be identified by name. The following statement assigns a name to resumable statements:

ALTER SESSION ENABLE RESUMABLE TIMEOUT 3600 NAME 'insert into table';

The NAME value remains in effect until it is changed by another ALTER SESSION ENABLE RESUMABLE statement, or the session ends. The default value for NAME is 'Userusername(userid), Sessionsessionid, Instanceinstanceid'.

The name of the statement is used to identify the resumable statement in the DBA_RESUMABLE and USER_RESUMABLE views.

Using a LOGON Trigger to Set Default Resumable Mode

Another method of setting default resumable mode, other than setting the RESUMABLE_TIMEOUT initialization parameter, is that you can register a database level LOGON trigger to alter a user's session to enable resumable and set a timeout interval.

Note:

If there are multiple triggers registered that change default mode and timeout for resumable statements, the result will be unspecified because Oracle Database does not guarantee the order of trigger invocation.

Detecting Suspended Statements

When a resumable statement is suspended, the error is not raised to the client. In order for corrective action to be taken, Oracle Database provides alternative methods for notifying users of the error and for providing information about the circumstances.

Notifying Users: The AFTER SUSPEND System Event and Trigger

When a resumable statement encounter a correctable error, the system internally generates the AFTER SUSPEND system event. Users can register triggers for this event at both the database and schema level. If a user registers a trigger to handle this system event, the trigger is executed after a SQL statement has been suspended.

SQL statements executed within a AFTER SUSPEND trigger are always non-resumable and are always autonomous. Transactions started within the trigger use the SYSTEM rollback segment. These conditions are imposed to overcome deadlocks and reduce the chance of the trigger experiencing the same error condition as the statement.

Users can use the USER_RESUMABLE or DBA_RESUMABLE views, or the DBMS_RESUMABLE.SPACE_ERROR_INFO function, within triggers to get information about the resumable statements.

Triggers can also call the DBMS_RESUMABLE package to terminate suspended statements and modify resumable timeout values. In the following example, the default system timeout is changed by creating a system wide AFTER SUSPEND trigger that calls DBMS_RESUMABLE to set the timeout to 3 hours:

Using Views to Obtain Information About Suspended Statements

The following views can be queried to obtain information about the status of resumable statements:

View

Description

DBA_RESUMABLE

USER_RESUMABLE

These views contain rows for all currently executing or suspended resumable statements. They can be used by a DBA, AFTER SUSPEND trigger, or another session to monitor the progress of, or obtain specific information about, resumable statements.

V$SESSION_WAIT

When a statement is suspended the session invoking the statement is put into a wait state. A row is inserted into this view for the session with the EVENT column containing "statement suspended, wait error to be cleared".

Using the DBMS_RESUMABLE Package

The DBMS_RESUMABLE package helps control resumable space allocation. The following procedures can be invoked:

Procedure

Description

ABORT(sessionID)

This procedure aborts a suspended resumable statement. The parameter sessionID is the session ID in which the statement is executing. For parallel DML/DDL, sessionID is any session ID which participates in the parallel DML/DDL.

Oracle Database guarantees that the ABORT operation always succeeds. It may be called either inside or outside of the AFTER SUSPEND trigger.

The caller of ABORT must be the owner of the session with sessionID, have ALTER SYSTEM privilege, or have DBA privileges.

GET_SESSION_TIMEOUT(sessionID)

This function returns the current timeout value of resumable space allocation for the session with sessionID. This returned timeout is in seconds. If the session does not exist, this function returns -1.

SET_SESSION_TIMEOUT(sessionID, timeout)

This procedure sets the timeout interval of resumable space allocation for the session with sessionID. The parameter timeout is in seconds. The new timeout setting will applies to the session immediately. If the session does not exist, no action is taken.

GET_TIMEOUT()

This function returns the current timeout value of resumable space allocation for the current session. The returned value is in seconds.

SET_TIMEOUT(timeout)

This procedure sets a timeout value for resumable space allocation for the current session. The parameter timeout is in seconds. The new timeout setting applies to the session immediately.

Operation-Suspended Alert

When a resumable session is suspended, an operation-suspended alert is issued on the object that needs allocation of resource for the operation to complete. Once the resource is allocated and the operation completes, the operation-suspended alert is cleared. See "Managing Tablespace Alerts" for more information on system-generated alerts.

In the following example, a system wide AFTER SUSPEND trigger is created and registered as user SYS at the database level. Whenever a resumable statement is suspended in any session, this trigger can have either of two effects:

If an undo segment has reached its space limit, then a message is sent to the DBA and the statement is aborted.

If any other recoverable error has occurred, the timeout interval is reset to 8 hours.

Reclaiming Wasted Space

This section explains how to reclaim wasted space, and also introduces the Segment Advisor, which is the Oracle Database component that identifies segments that have space available for reclamation. The following topics are covered:

Understanding Reclaimable Unused Space

Over time, updates and deletes on objects within a tablespace can create pockets of empty space that individually are not large enough to be reused for new data. This type of empty space is referred to as fragmented free space.

Objects with fragmented free space can result in much wasted space, and can impact database performance. The preferred way to defragment and reclaim this space is to perform an online segment shrink. This process consolidates fragmented free space below the high water mark and compacts the segment. After compaction, the high water mark is moved, resulting in new free space above the high water mark. That space above the high water mark is then deallocated. The segment remains available for queries and DML during most of the operation, and no extra disk space need be allocated.

You use the Segment Advisor to identify segments that would benefit from online segment shrink. Only segments in locally managed tablespaces with automatic segment space management (ASSM) are eligible. Other restrictions on segment type exist. For more information, see "Shrinking Database Segments Online".

If a table with reclaimable space is not eligible for online segment shrink, or if you want to make changes to logical or physical attributes of the table while reclaiming space, then you can use online table redefinition as an alternative to segment shrink. Online redefinition is also referred to as reorganization. Unlike online segment shrink, it requires extra disk space to be allocated. See "Redefining Tables Online" for more information.

Using the Segment Advisor

The Segment Advisor identifies segments that have space available for reclamation. It performs its analysis by examining usage and growth statistics in the Automatic Workload Repository (AWR), and by sampling the data in the segment. It is configured to run during maintenance windows as an automated maintenance task, and you can also run it on demand (manually). The Segment Advisor automated maintenance task is known as the Automatic Segment Advisor.

The Segment Advisor generates the following types of advice:

If the Segment Advisor determines that an object has a significant amount of free space, it recommends online segment shrink. If the object is a table that is not eligible for shrinking, as in the case of a table in a tablespace without automatic segment space management, the Segment Advisor recommends online table redefinition.

If the Segment Advisor determines that a table could benefit from compression with the OLTP compression method, it makes a recommendation to that effect. (Automatic Segment Advisor only. See "Automatic Segment Advisor".)

If the Segment Advisor encounters a table with row chaining above a certain threshold, it records that fact that the table has an excess of chained rows.

Note:

The Segment Advisor flags only the type of row chaining that results from updates that increase row length.

If you receive a space management alert, or if you decide that you want to reclaim space, you should start with the Segment Advisor.

Automatic Segment Advisor

The Automatic Segment Advisor is an automated maintenance task that is configured to run during all maintenance windows.

The Automatic Segment Advisor does not analyze every database object. Instead, it examines database statistics, samples segment data, and then selects the following objects to analyze:

Tablespaces that have exceeded a critical or warning space threshold

Segments that have the most activity

Segments that have the highest growth rate

In addition, the Automatic Segment Advisor evaluates tables that are at least 10MB and that have at least three indexes to determine the amount of space saved if the tables are compressed with the OLTP compression method.

If an object is selected for analysis but the maintenance window expires before the Segment Advisor can process the object, the object is included in the next Automatic Segment Advisor run.

You cannot change the set of tablespaces and segments that the Automatic Segment Advisor selects for analysis. You can, however, enable or disable the Automatic Segment Advisor task, change the times during which the Automatic Segment Advisor is scheduled to run, or adjust automated maintenance task system resource utilization. See "Configuring the Automatic Segment Advisor" for more information.

Running the Segment Advisor Manually

You can manually run the Segment Advisor at any time with Enterprise Manager or with PL/SQL package procedure calls. Reasons to manually run the Segment Advisor include the following:

You want to analyze a tablespace or segment that was not selected by the Automatic Segment Advisor.

You want to repeat the analysis of an individual tablespace or segment to get more up-to-date recommendations.

You can request advice from the Segment Advisor at three levels:

Segment level—Advice is generated for a single segment, such as an unpartitioned table, a partition or subpartition of a partitioned table, an index, or a LOB column.

Object level—Advice is generated for an entire object, such as a table or index. If the object is partitioned, advice is generated on all the partitions of the object. In addition, if you run Segment Advisor manually from Enterprise Manager, you can request advice on the object's dependent objects, such as indexes and LOB segments for a table.

Tablespace level—Advice is generated for every segment in a tablespace.

The OBJECT_TYPE column of Table 19-2 shows the types of objects for which you can request advice.

Running the Segment Advisor Manually with Enterprise Manager

You must have the OEM_ADVISOR role to run the Segment Advisor manually with Enterprise Manager. There are two ways to run the Segment Advisor:

Using the Segment Advisor Wizard

This method enables you to request advice at the tablespace level or object level. At the object level, you can request advice on tables, indexes, table partitions, and index partitions.

Using the Run Segment Advisor command on a schema object page.

For example, if you display a list of tables on the Tables page (accessible from the Schema page), you can select a table and then select the Run Segment Advisor command from the Actions menu.

This method enables you to include the schema object's dependent objects in the Segment Advisor run. For example, if you select a table and select the Run Segment Advisor command, Enterprise Manager displays the table's dependent objects, such as partitions, index segments, LOB segments, and so on. You can then select dependent objects to include in the run.

In both cases, Enterprise Manager creates the Segment Advisor task as an Oracle Database Scheduler job. You can schedule the job to run immediately, or can take advantage of advanced scheduling features offered by the Scheduler.

To run the Segment Advisor manually with the Segment Advisor Wizard:

From the database Home page, under Related Links, click Advisor Central.

Follow the wizard steps to schedule the Segment Advisor job, and then click Submit on the final wizard page.

The Advisor Central page reappears, with the new Segment Advisor job at the top of the list under the Results heading. The job status is SCHEDULED or RUNNING. (If you do not see your job, use the search fields above the list to display it.)

Check the status of the job. If it is not COMPLETED, click the Refresh button at the top of the page repeatedly. (Do not use your browser's Refresh icon.)

When the job status changes to COMPLETED, select the job by clicking in the Select column, and then click View Result.

Running the Segment Advisor Manually with PL/SQL

You can also run the Segment Advisor with the DBMS_ADVISOR package. You use package procedures to create a Segment Advisor task, set task arguments, and then execute the task. You must have the ADVISOR privilege. Table 19-1 shows the procedures that are relevant for the Segment Advisor. See Oracle Database PL/SQL Packages and Types Reference for more details on these procedures.

Use this procedure to create the Segment Advisor task. Specify 'Segment Advisor' as the value of the ADVISOR_NAME parameter.

CREATE_OBJECT

Use this procedure to identify the target object for segment space advice. The parameter values of this procedure depend upon the object type. Table 19-2 lists the parameter values for each type of object.

Note: To request advice on an IOT overflow segment, use an object type of TABLE, TABLEPARTITION, or TABLESUBPARTITION. Use the following query to find the overflow segment for an IOT and to determine the overflow segment table name to use with CREATE_OBJECT:

select table_name, iot_name, iot_type from dba_tables;

SET_TASK_PARAMETER

Use this procedure to describe the segment advice that you need. Table 19-3 shows the relevant input parameters of this procedure. Parameters not listed here are not used by the Segment Advisor.

EXECUTE_TASK

Use this procedure to execute the Segment Advisor task.

Table 19-2 Input for DBMS_ADVISOR.CREATE_OBJECT

Input Parameter

OBJECT_TYPE

ATTR1

ATTR2

ATTR3

ATTR4

TABLESPACE

tablespace name

NULL

NULL

Unused. Specify NULL.

TABLE

schema name

table name

NULL

Unused. Specify NULL.

INDEX

schema name

index name

NULL

Unused. Specify NULL.

TABLE PARTITION

schema name

table name

table partition name

Unused. Specify NULL.

INDEX PARTITION

schema name

index name

index partition name

Unused. Specify NULL.

TABLE SUBPARTITION

schema name

table name

table subpartition name

Unused. Specify NULL.

INDEX SUBPARTITION

schema name

index name

index subpartition name

Unused. Specify NULL.

LOB

schema name

segment name

NULL

Unused. Specify NULL.

LOB PARTITION

schema name

segment name

lob partition name

Unused. Specify NULL.

LOB SUBPARTITION

schema name

segment name

lob subpartition name

Unused. Specify NULL.

Table 19-3 Input for DBMS_ADVISOR.SET_TASK_PARAMETER

Input Parameter

Description

Possible Values

Default Value

time_limit

The time limit for the Segment Advisor run, specified in seconds.

Any number of seconds

UNLIMITED

recommend_all

Whether the Segment Advisor should generate findings for all segments.

TRUE: Findings are generated on all segments specified, whether or not space reclamation is recommended.

FALSE: Findings are generated only for those objects that generate recommendations for space reclamation.

TRUE

Example The example that follows shows how to use the DBMS_ADVISOR procedures to run the Segment Advisor for the sample table hr.employees. The user executing these package procedures must have the EXECUTE object privilege on the package or the ADVISOR system privilege.

Note that passing an object type of TABLE to DBMS_ADVISOR.CREATE_OBJECT amounts to an object level request. If the table is not partitioned, the table segment is analyzed (without any dependent segments like index or LOB segments). If the table is partitioned, the Segment Advisor analyzes all table partitions and generates separate findings and recommendations for each.

Viewing Segment Advisor Results

The Segment Advisor creates several types of results: recommendations, findings, actions, and objects. You can view results in the following ways:

With Enterprise Manager

By querying the DBA_ADVISOR_* views

By calling the DBMS_SPACE.ASA_RECOMMENDATIONS procedure

Table Table 19-4 describes the various result types and their associated DBA_ADVISOR_* views.

Table 19-4 Segment Advisor Result Types

Result Type

Associated View

Description

Recommendations

DBA_ADVISOR_RECOMMENDATIONS

If a segment would benefit from a segment shrink, reorganization, or compression, the Segment Advisor generates a recommendation for the segment. Table 19-5 shows examples of generated findings and recommendations.

Findings

DBA_ADVISOR_FINDINGS

Findings are a report of what the Segment Advisor observed in analyzed segments. Findings include space used and free space statistics for each analyzed segment. Not all findings result in a recommendation. (There may be only a few recommendations, but there could be many findings.) When running the Segment Advisor manually with PL/SQL, if you specify 'TRUE' for recommend_all in the SET_TASK_PARAMETER procedure, then the Segment Advisor generates a finding for each segment that qualifies for analysis, whether or not a recommendation is made for that segment. For row chaining advice, the Automatic Segment Advisor generates findings only, and not recommendations. If the Automatic Segment Advisor has no space reclamation recommendations to make, it does not generate findings. However, the Automatic Segment Advisor may generate findings for tables that could benefit from OLTP compression.

Actions

DBA_ADVISOR_ACTIONS

Every recommendation is associated with a suggested action to perform: either segment shrink, online redefinition (reorganization), or compression. The DBA_ADVISOR_ACTIONS view provides either the SQL that you can use to perform a segment shrink or table compression, or a suggestion to reorganize the object.

Objects

DBA_ADVISOR_OBJECTS

All findings, recommendations, and actions are associated with an object. If the Segment Advisor analyzes multiple segments, as with a tablespace or partitioned table, then one entry is created in the DBA_ADVISOR_OBJECTS view for each analyzed segment. Table 19-2 defines the columns in this view to query for information on the analyzed segments. You can correlate the objects in this view with the objects in the findings, recommendations, and actions views.

If any recommendations are present, select a tablespace and click Recommendation Details.

The Recommendation Details page appears. (See Figure 19-4.) You can initiate the recommended activity from this page (shrink or reorganize).

Note:

Dependent objects, such as LOBs, are not supported using the provided Shrink option. Instead, use Segment Advisor to confirm reclaimable space, but perform the shrink operation manually.

To view Segment Advisor results with Enterprise Manager—Specific run:

Start at the Advisor Central page.

If you ran the Segment Advisor with the Enterprise Manager wizard, the Advisor Central page appears after you submit the Segment Advisor task. Otherwise, to get to this page, on the Database Home page, under Related Links, click Advisor Central.

Check that your task appears in the list under the Results heading. If it does not, complete these steps (See Figure 19-2):

In the Search section of the page, under Advisor Tasks, select Segment Advisor in the Advisory Type list.

In the Advisor Runs list, select All or the desired time period.

(Optional) Enter a task name.

Click Go.

Your Segment Advisor task appears in the Results section.

Check the status of the job. If it is not COMPLETED, click the Refresh button at the top of the page until your task status shows COMPLETED. (Do not use your browser's refresh icon.)

The Chained Row Analysis page appears, showing all segments that have chained rows, with a chained rows percentage for each.

Viewing Segment Advisor Results by Querying the DBA_ADVISOR_* Views

The headings of Table 19-5 show the columns in the DBA_ADVISOR_* views that contain output from the Segment Advisor. See Oracle Database Reference for a description of these views. The table contents summarize the possible outcomes. In addition, Table 19-2 defines the columns in the DBA_ADVISOR_OBJECTS view that contain information on the analyzed segments.

Before querying the DBA_ADVISOR_* views, you can check that the Segment Advisor task is complete by querying the STATUS column in DBA_ADVISOR_TASKS.

(Note: This finding is for objects with reclaimable space that are not eligible for online segment shrink.)

Allocated Space:xxx: Used Space:xxx: Reclaimable Space :xxx

Perform re-org on the object object, estimated savings is xxx bytes.

Perform reorg

The object has chained rows that can be removed by re-org.

xx percent chained rows can be removed by re-org.

-

-

Compress object object_name, estimated savings is xxx bytes.

(This outcome is generated by the Automatic Segment Advisor only)

Compress object object_name, estimated savings is xxx bytes.

-

The command to execute. For example: ALTERTABLET1COMPRESSFOROLTP

For this finding, see also the ATTR2 column of DBA_ADVISOR_ACTIONS.

Viewing Segment Advisor Results with DBMS_SPACE.ASA_RECOMMENDATIONS

The ASA_RECOMMENDATIONS procedure in the DBMS_SPACE package returns a nested table object that contains findings or recommendations for Automatic Segment Advisor runs and, optionally, manual Segment Advisor runs. Calling this procedure may be easier than working with the DBA_ADVISOR_* views, because the procedure performs all the required joins for you and returns information in an easily consumable format.

The following query returns recommendations by the most recent run of the Auto Segment Advisor, with the suggested command to run to follow the recommendations:

Configuring the Automatic Segment Advisor

The Automatic Segment Advisor is an automated maintenance task. As such, you can use Enterprise Manager or PL/SQL package procedure calls to modify when (and if) this task runs. You can also control the resources allotted to it by modifying the appropriate resource plans.

You can call PL/SQL package procedures to make these changes, but the easier way to is to use Enterprise Manager.

To configure the Automatic Segment Advisor task with Enterprise Manager:

Log in to Enterprise Manager as user SYSTEM.

On the Database Home page, under the Space Summary heading, click the numeric link next to the label Segment Advisor Recommendations.

Viewing Automatic Segment Advisor Information

The following views display information specific to the Automatic Segment Advisor. For details, see Oracle Database Reference.

View

Description

DBA_AUTO_SEGADV_SUMMARY

Each row of this view summarizes one Automatic Segment Advisor run. Fields include number of tablespaces and segments processed, and number of recommendations made.

DBA_AUTO_SEGADV_CTL

Contains control information that the Automatic Segment Advisor uses to select and process segments. Each row contains information on a single object (tablespace or segment), including whether the object has been processed, and if so, the task ID under which it was processed and the reason for selecting it.

Shrinking Database Segments Online

You use online segment shrink to reclaim fragmented free space below the high water mark in an Oracle Database segment. The benefits of segment shrink are these:

The compacted data requires fewer blocks to be scanned in full table scans, which in turns leads to better decision support system (DSS) performance.

Segment shrink is an online, in-place operation. DML operations and queries can be issued during the data movement phase of segment shrink. Concurrent DML operations are blocked for a short time at the end of the shrink operation, when the space is deallocated. Indexes are maintained during the shrink operation and remain usable after the operation is complete. Segment shrink does not require extra disk space to be allocated.

Segment shrink reclaims unused space both above and below the high water mark. In contrast, space deallocation reclaims unused space only above the high water mark. In shrink operations, by default, the database compacts the segment, adjusts the high water mark, and releases the reclaimed space.

Segment shrink requires that rows be moved to new locations. Therefore, you must first enable row movement in the object you want to shrink and disable any rowid-based triggers defined on the object. You enable row movement in a table with the ALTERTABLE ... ENABLEROWMOVEMENT command.

Shrink operations can be performed only on segments in locally managed tablespaces with automatic segment space management (ASSM). Within an ASSM tablespace, all segment types are eligible for online segment shrink except these:

Before invoking online segment shrink, view the findings and recommendations of the Segment Advisor. For more information, see "Using the Segment Advisor".

You invoke online segment shrink with Oracle Enterprise Manager or with SQL commands in SQL*Plus. The remainder of this section discusses the command line method.

Note:

You can invoke segment shrink directly from the Recommendation Details page in Enterprise Manager. (See Figure 19-4.) Or, to invoke segment shrink for an individual table in Enterprise Manager, display the table on the Tables page, select the table, and then click Shrink Segment in the Actions list. (See Figure 19-1.) Perform a similar operation in Enterprise Manager to shrink indexes, materialized views, and so on.

You can shrink space in a table, index-organized table, index, partition, subpartition, materialized view, or materialized view log. You do this using ALTERTABLE, ALTERINDEX, ALTERMATERIALIZEDVIEW, or ALTERMATERIALIZEDVIEWLOG statement with the SHRINK SPACE clause.

Two optional clauses let you control how the shrink operation proceeds:

The COMPACT clause lets you divide the shrink segment operation into two phases. When you specify COMPACT, Oracle Database defragments the segment space and compacts the table rows but postpones the resetting of the high water mark and the deallocation of the space until a future time. This option is useful if you have long-running queries that might span the operation and attempt to read from blocks that have been reclaimed. The defragmentation and compaction results are saved to disk, so the data movement does not have to be redone during the second phase. You can reissue the SHRINK SPACE clause without the COMPACT clause during off-peak hours to complete the second phase.

The CASCADE clause extends the segment shrink operation to all dependent segments of the object. For example, if you specify CASCADE when shrinking a table segment, all indexes of the table will also be shrunk. (You need not specify CASCADE to shrink the partitions of a partitioned table.) To see a list of dependent segments of a given object, you can run the OBJECT_DEPENDENT_SEGMENTS procedure of the DBMS_SPACE package.

As with other DDL operations, segment shrink causes subsequent SQL statements to be reparsed because of invalidation of cursors unless you specify the COMPACT clause.

Examples

Shrink a table and all of its dependent segments (including BASICFILE LOB segments):

ALTER TABLE employees SHRINK SPACE CASCADE;

Shrink a BASICFILE LOB segment only:

ALTER TABLE employees MODIFY LOB (perf_review) (SHRINK SPACE);

Shrink a single partition of a partitioned table:

ALTER TABLE customers MODIFY PARTITION cust_P1 SHRINK SPACE;

Shrink an IOT index segment and the overflow segment:

ALTER TABLE cities SHRINK SPACE CASCADE;

Shrink an IOT overflow segment only:

ALTER TABLE cities OVERFLOW SHRINK SPACE;

See Also:

Oracle Database SQL Language Reference for the syntax and restrictions of the ALTERTABLE, ALTERINDEX, ALTERMATERIALIZEDVIEW, and ALTERMATERIALIZEDVIEWLOG statements with the SHRINK SPACE clause

Deallocating Unused Space

When you deallocate unused space, the database frees the unused space at the unused (high water mark) end of the database segment and makes the space available for other segments in the tablespace.

Prior to deallocation, you can run the UNUSED_SPACE procedure of the DBMS_SPACE package, which returns information about the position of the high water mark and the amount of unused space in a segment. For segments in locally managed tablespaces with automatic segment space management, use the SPACE_USAGE procedure for more accurate information on unused space.

Dropping Unused Object Storage

Beginning with Oracle Database 11g release 2 (11.2.0.2), the DBMS_SPACE_ADMIN package includes the DROP_EMPTY_SEGMENTS procedure, which enables you to drop segments for empty tables and partitions that have been migrated from previous releases. This includes segments of dependent objects of the table, such as index segments, where possible.

The following example drops empty segments from every table in the database.

BEGIN
DBMS_SPACE_ADMIN.DROP_EMPTY_SEGMENTS();
END;

The following drops empty segments from the HR.EMPLOYEES table, including dependent objects.

Displaying Information About Space Usage for Schema Objects

Oracle Database provides data dictionary views and PL/SQL packages that allow you to display information about the space usage of schema objects. Views and packages that are unique to a particular schema object are described in the chapter of this book associated with that object. This section describes views and packages that are generic in nature and apply to multiple schema objects.

Example 2: Displaying Extent Information

Information about the currently allocated extents in a database is stored in the DBA_EXTENTS data dictionary view. For example, the following query identifies the extents allocated to each index segment in the hr schema and the size of each of those extents:

Example 3: Displaying the Free Space (Extents) in a Tablespace

Information about the free extents (extents not allocated to any segment) in a database is stored in the DBA_FREE_SPACE data dictionary view. For example, the following query reveals the amount of free space available as free extents in the SMUNDO tablespace:

Capacity Planning for Database Objects

This section discusses the PL/SQL method. See Enterprise Manager online help and Oracle Database 2 Day DBA for details on capacity planning with Enterprise Manager.

Three procedures in the DBMS_SPACE package enable you to predict the size of new objects and monitor the size of existing database objects. This section discusses those procedures and contains the following sections:

Estimating the Space Use of a Table

The size of a database table can vary greatly depending on tablespace storage attributes, tablespace block size, and many other factors. The CREATE_TABLE_COST procedure of the DBMS_SPACE package lets you estimate the space use cost of creating a table. See Oracle Database PL/SQL Packages and Types Reference for details on the parameters of this procedure.

The procedure has two variants. The first variant uses average row size to estimate size. The second variant uses column information to estimate table size. Both variants require as input the following values:

TABLESPACE_NAME: The tablespace in which the object will be created. The default is the SYSTEM tablespace.

ROW_COUNT: The anticipated number of rows in the table.

PCT_FREE: The percentage of free space you want to reserve in each block for future expansion of existing rows due to updates.

In addition, the first variant also requires as input a value for AVG_ROW_SIZE, which is the anticipated average row size in bytes.

The second variant also requires for each anticipated column values for COLINFOS, which is an object type comprising the attributes COL_TYPE (the data type of the column) and COL_SIZE (the number of characters or bytes in the column).

The procedure returns two values:

USED_BYTES: The actual bytes used by the data, including overhead for block metadata, PCT_FREE space, and so forth.

ALLOC_BYTES: The amount of space anticipated to be allocated for the object taking into account the tablespace extent characteristics.

Note:

Beginning with release 11.2.0.2 of Oracle Database, the default size of the first extent of any new segment for a partitioned table is 8 MB instead of 64 KB. This helps improve performance of inserts and queries on partitioned tables. Although partitioned tables will start with a larger initial size, once sufficient data is inserted, the space consumption will be the same as in previous releases. You can override this default by setting the INITIAL size in the storage clause for the table. This new default only applies to table partitions and LOB partitions.

Estimating the Space Use of an Index

The CREATE_INDEX_COST procedure of the DBMS_SPACE package lets you estimate the space use cost of creating an index on an existing table.

The procedure requires as input the following values:

DDL: The CREATE INDEX statement that would create the index. The table specified in this DDL statement must be an existing table.

[Optional] PLAN_TABLE: The name of the plan table to use. The default is NULL.

The results returned by this procedure depend on statistics gathered on the segment. Therefore, be sure to obtain statistics shortly before executing this procedure. In the absence of recent statistics, the procedure does not issue an error, but it may return inappropriate results. The procedure returns the following values:

USED_BYTES: The number of bytes representing the actual index data.

ALLOC_BYTES: The amount of space allocated for the index in the tablespace.

Obtaining Object Growth Trends

The OBJECT_GROWTH_TREND function of the DBMS_SPACE package produces a table of one or more rows, where each row describes the space use of the object at a specific time. The function retrieves the space use totals from the Automatic Workload Repository or computes current space use and combines it with historic space use changes retrieved from Automatic Workload Repository. See Oracle Database PL/SQL Packages and Types Reference for detailed information on the parameters of this function.

The function requires as input the following values:

OBJECT_OWNER: The owner of the object.

OBJECT_NAME: The name of the object.

PARTITION_NAME: The name of the table or index partition, is relevant. Specify NULL otherwise.

OBJECT_TYPE: The type of the object.

START_TIME: A TIMESTAMP value indicating the beginning of the growth trend analysis.

END_TIME: A TIMESTAMP value indicating the end of the growth trend analysis. The default is "NOW".

INTERVAL: The length in minutes of the reporting interval during which the function should retrieve space use information.

SKIP_INTERPOLATED: Determines whether the function should omit values based on recorded statistics before and after the INTERVAL ('YES') or not ('NO'). This setting is useful when the result table will be displayed as a table rather than a chart, because you can see more clearly how the actual recording interval relates to the requested reporting interval.

The function returns a table, each of row of which provides space use information on the object for one interval. If the return table is very large, the results are pipelined so that another application can consume the information as it is being produced. The output table has the following columns:

TIMEPOINT: A TIMESTAMP value indicating the time of the reporting interval.

Records are not produced for values of TIME that precede the oldest recorded statistics for the object.

SPACE_USAGE: The number of bytes actually being used by the object data.

SPACE_ALLOC: The number of bytes allocated to the object in the tablespace at that time.

QUALITY: A value indicating how well the requested reporting interval matches the actual recording of statistics. This information is useful because there is no guaranteed reporting interval for object size use statistics, and the actual reporting interval varies over time and from object to object.

The values of the QUALITY column are:

GOOD: The value whenever the value of TIME is based on recorded statistics with a recorded timestamp within 10% of the INTERVAL specified in the input parameters.

INTERPOLATED: The value did not meet the criteria for GOOD, but was based on recorded statistics before and after the value of TIME. Current in-memory statistics can be collected across all instances in a cluster and treated as the "recorded" value for the present time.

PROJECTION: The value of TIME is in the future as of the time the table was produced. In an Oracle Real Application Clusters environment, the rules for recording statistics allow each instance to choose independently which objects will be selected.

The output returned by this function is an aggregation of values recorded across all instances in an Oracle RAC environment. Each value can be computed from a combination of GOOD and INTERPOLATED values. The aggregate value returned is marked GOOD if at least 80% of that value was derived from GOOD instance values.

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